Comparative Study of α- and β-MnO2 on Methyl Mercaptan Decomposition: The Role of Oxygen Vacancies

As a representative sulfur-containing volatile organic compounds (S-VOCs), CH3SH has attracted widespread attention due to its adverse environmental and health risks. The performance of Mn-based catalysts and the effect of their crystal structure on the CH3SH catalytic reaction have yet to be systematically investigated. In this paper, two different crystalline phases of tunneled MnO2 (α-MnO2 and β-MnO2) with the similar nanorod morphology were used to remove CH3SH, and their physicochemical properties were comprehensively studied using high-resolution transmission electron microscope (HRTEM) and electron paramagnetic resonance (EPR), H2-TPR, O2-TPD, Raman, and X-ray photoelectron spectroscopy (XPS) analysis. For the first time, we report that the specific reaction rate for α-MnO2 (0.029 mol g−1 h−1) was approximately 4.1 times higher than that of β-MnO2 (0.007 mol g−1 h−1). The as-synthesized α-MnO2 exhibited higher CH3SH catalytic activity towards CH3SH than that of β-MnO2, which can be ascribed to the additional oxygen vacancies, stronger surface oxygen migration ability, and better redox properties from α-MnO2. The oxygen vacancies on the catalyst surface provided the main active sites for the chemisorption of CH3SH, and the subsequent electron transfer led to the decomposition of CH3SH. The lattice oxygen on catalysts could be released during the reaction and thus participated in the further oxidation of sulfur-containing species. CH3SSCH3, S0, SO32−, and SO42− were identified as the main products of CH3SH conversion. This work offers a new understanding of the interface interaction mechanism between Mn-based catalysts and S-VOCs.


Introduction
As a particular class of volatile organic compounds (VOCs), sulfur-containing volatile organic compounds (S-VOCs) can be converted into sulfate aerosols in the atmosphere through complex physicochemical reactions [1]. They can also react indirectly with NO x through photochemistry reactions, which are the crucial precursors for forming PM 2.5 and O 3 . Methyl mercaptan (CH 3 SH), a representative S-VOC, is considered as an important air odor pollutant, which is harmful to the ecosystem and human health owing to its severe toxicity and low olfactory threshold [2][3][4]. In previous studies, various methods have been employed to eliminate CH 3 SH, such as adsorption [5,6], biodegradation [7], photocatalytic oxidation [8], and catalytic oxidation [9,10]. However, these remediation technologies suffer from secondary pollution because of incomplete removal and high cost. Until now, catalytic decomposition has been regarded as the most promising strategy for removing S-VOCs Herein, we compared the removal efficiency of CH 3 SH by α-MnO 2 and β-MnO 2 with similar surface morphology but different crystal structures. Their physicochemical properties were subsequently characterized by various analysis techniques. The number of surface low valence Mn, oxygen vacancies and redox properties were studied regarding high-resolution transmission electron microscope (HRTEM) and electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), H 2 -TPR and O 2 -TPD. The changes of catalyst surface species before and after the reaction were characterized by XPS, and the variation of intermediate species of CH 3 SH during the reaction were also monitored.
Synthesis of α-MnO 2 : 0.1 M KMnO 4 and 0.05 M MnSO 4 ·H 2 O were dissolved in 70 mL deionized water and stirred for 30 min. The resulting solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 160 • C for 12 h. After cooling to room temperature, the precipitate was centrifuged and washed with distilled water (700-1000 mL) three times. Finally, the precipitate was dried at 80 • C for 4 h and calcination at 360 • C for 2 h.
Synthesis of β-MnO 2 : 0.14 M MnSO 4 ·H 2 O and 0.14 M (NH 4 ) 2 S 2 O 8 were dissolved in 70 mL deionized water and stirred for 30 min. The resulting solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 140 • C for 12 h. After cooling to room temperature, the precipitate was centrifuged and washed with distilled water (700-1000 mL) three times. Finally, the precipitate was dried at 80 • C for 4 h and calcination at 360 • C for 2 h.

Catalyst Characterization
The refined test of X-ray powder diffraction (XRD) of the products was performed using a Bragg-Brentano-type powder diffractometer (Nihongo TTRIII, Tokyo City, Japan, operated at 40 kV and 200 mA, Cu Kα radiation, λ = 0.15418 nm). To investigate the Brunauer-Emmett-Teller (BET) surface areas, average pore diameters, and total pore volumes of the samples, N 2 adsorption-desorption isotherms were determined using a NOVA 4200e Surface Area and Pore Size Analyzer. Electron paramagnetic resonance (EPR) signals were carried out on a Bruker A300 spectrometer (Saarbrucken, Germany) at 25 • C. XPS profiles were obtained with a Thermo Scientific K-Alpha spectrometer (Waltham, MA, USA). The binding energy (BE) values were calibrated using the C 1 s peak at 284.8 eV. The Raman spectra were recorded using a 514 nm laser excitation source with an integration time of 3 s and 30 accumulations (Raman, BX41, HOEIBA Scientific, Paris, France). Scanning electron microscopy (SEM, VEGA3SBH, Brno, Czech Republic) and high-resolution transmission electron microscopy (HRTEM, Talos F200X, Thermo Scientific, Waltham, MA, USA) were used to observe catalyst morphologies.
Hydrogen temperature-programmed reduction (H 2 -TPR) and oxygen temperatureprogrammed desorption (O 2 -TPD) experiments were performed on a FULI II 7970 gas chromatograph (Fuli Analytical Instrument Inc., Hangzhou, China) with a thermal conductivity detector (TCD). In H 2 -TPR experiments, 50 mg of the sample was placed in a quartz tube and pretreated in a gas flow of 10% H 2 /Ar (30 mL min −1 ) at 100 • C for 30 min to remove impurities. After the pretreatment process, the sample was reduced by 10% H 2 /Ar (30 mL min −1 ) from 100 to 800 • C with a heating rate of 10 • C/min. For O 2 -TPD analysis, 50 mg of sample was loaded on the quartz tube, heated to 105 • C and pretreated with He (30 mL min −1 ) for 30 min to remove surface adsorbed water, followed by cooling to 30 • C. Subsequently, the sample was adsorbed by 10% O 2 /He (30 mL min −1 ) at room temperature for 60 min, and then He (30 mL min −1 ) was used to purge the sample for Nanomaterials 2023, 13, 775 4 of 16 30 min to remove physically adsorbed O 2 and stabilize the baseline. Subsequently, the temperature was ramped from 30 to 850 • C at 10 • C mL min −1 .

Catalyst Activity Evaluation
The catalytic performance for the CH 3 SH decomposition was investigated in a fixedbed quartz tube reactor (i.d. = 6 mm). 200 mg samples with the size of 40-60 meshes were loaded into the reactor. The reaction temperature was controlled and maintained for about 1 h at each designated temperature. The inlet CH 3 SH concentration was set at 5000 ppm, and the total flow rate was maintained at 30 mL min −1 . The concentration of CH 3 SH was recorded by GC-9790 (FULI, China) equipped with a flame ionization detector (FID) and flame photometric detector (FPD), and the CH 3 SH conversion ratio was calculated as follows: C in represents the inlet concentration of CH 3 SH and C out is the outlet concentration of CH 3 SH.
The reaction rates of CH 3 SH decomposition were determined in the kinetic regime at a CH 3 SH conversion lower than 20% at different temperatures; The reaction rate (r CH3SH ; mol g −1 h −1 ) for CH 3 SH decomposition was calculated according to the following equations: where the C CH 3 SH represents the initial methyl mercaptan concentration, F (mol·h −1 ) represents the total flow rate, X CH 3 SH denotes CH 3 SH conversion, and S BET (m 2 ·g −1 ) represents the specific surface area of catalysts. The turnover frequency (TOF, h −1 ) was calculated for different crystal types based on oxygen vacancy concentration for MnO 2 , and indicates the number of reactions of methyl mercaptan at each active site per unit of time, thus TOF was obtained using the following equation: is the molar mass of MnO 2 , and Mn 2+ + Mn 3+ derived from XPS data, which represent the concentration of the oxygen vacancies of MnO 2 deduced from the obtained XPS spectra.

Structure and Morphology
XRD was used to determine the crystal structure of the prepared material. The XRD patterns of as-prepared MnO 2 with various crystal types are shown in Figure 1. The diffraction peaks located at~12.7 • ,~18.1 • ,~28.8 • , and~37.5 • can be assigned to α-MnO 2 (JCPDS card no. 44-0141) ( Figure 1A), and the peaks at~28.7 • ,~37.2 • ,~42.7 • and~56.4 • can be ascribed to β-MnO 2 (JCPDS card no. 24-0735) ( Figure 1B) [29]. There was no apparent crystal transformation on these samples after 360 • C calcination. The sharp and strait peaks of β-MnO 2 could indicate its great crystallization and large grain size; α-MnO 2 presented wide bands with relatively lower crystallinity and smaller grain sizes. The above results indicate the successful obtaining of the two kinds of MnO 2 with specific crystal phases.  The morphologies of MnO2 samples were characterized by a scanning electron microscopy (SEM), a transmission electron microscope (TEM), and a high-resolution TEM (HRTEM). As can be seen in Figure 2A, α-MnO2 showed a stacking-nanorod structure with an average length of about 330 nm. β-MnO2 ( Figure 2F) also showed a typical rod shape with a diameter near 50 nm and a length of ~1.2 μm. TEM showed consistent results with the SEM that α-MnO2 ( Figure 2B) and β-MnO2 ( Figure 2G) had similar nanorod-like structures as previously reported [30]. The well-identified periodic lattice fringes of 6.9 Å can be clearly observed in Figure 2C, corresponding to the interplanar distance of (110) facet of α-MnO2. Figure 2H exhibited the lattice fringes of 3.1 Å, which match the interplanar distance of the (110) facet of β-MnO2 well. Compared with the β-MnO2 samples, α-MnO2 showed more blurry lattice fringes, representing poor crystallinity, which also agrees well with the XRD patterns. In addition, the presence of defects was further demonstrated using the inverse Fast Fourier Transform (FFT) pattern ( Figure 2E,J). Significantly more lattice distortion can be clearly observed on the surface of α-MnO2 ( Figure 2D) (highlighted by red ovals), thus leading to more defects than β-MnO2 [10]. Besides, severe blurring of the lattice fringes was also detected on α-MnO2 than β-MnO2. Lattice distortion can be caused by nearby point defects. Simultaneously, a defect layer will be formed once the defect concentration is high enough, resulting in a blurry lattice fringe in the HRTEM images [31]. Hence, the intrinsic defective structure of α-MnO2 was confirmed. Oxygen vacancies, as an important point defect in catalysts, play a prominent role in the catalytic reaction process, and the high oxygen vacancy concentration will result in a blurry lattice fringe, which can be reflected in the HRTEM images [32]. As shown in Figure 3, the EPR signal corresponding to g = 2.003 can be attributed to oxygen vacancies, and its signal intensity can represent the number of oxygen vacancies [27]. Therefore, more oxygen vacancies on α-MnO2 than β-MnO2 can be confirmed based on EPR, consistent with HRTEM analysis.  The morphologies of MnO 2 samples were characterized by a scanning electron microscopy (SEM), a transmission electron microscope (TEM), and a high-resolution TEM (HRTEM). As can be seen in Figure 2A, α-MnO 2 showed a stacking-nanorod structure with an average length of about 330 nm. β-MnO 2 ( Figure 2F) also showed a typical rod shape with a diameter near 50 nm and a length of~1.2 µm. TEM showed consistent results with the SEM that α-MnO 2 ( Figure 2B) and β-MnO 2 ( Figure 2G) had similar nanorod-like structures as previously reported [30]. The well-identified periodic lattice fringes of 6.9 Å can be clearly observed in Figure 2C, corresponding to the interplanar distance of (110) facet of α-MnO 2 . Figure 2H exhibited the lattice fringes of 3.1 Å, which match the interplanar distance of the (110) facet of β-MnO 2 well. Compared with the β-MnO 2 samples, α-MnO 2 showed more blurry lattice fringes, representing poor crystallinity, which also agrees well with the XRD patterns. In addition, the presence of defects was further demonstrated using the inverse Fast Fourier Transform (FFT) pattern ( Figure 2E,J). Significantly more lattice distortion can be clearly observed on the surface of α-MnO 2 ( Figure 2D) (highlighted by red ovals), thus leading to more defects than β-MnO 2 [10]. Besides, severe blurring of the lattice fringes was also detected on α-MnO 2 than β-MnO 2 . Lattice distortion can be caused by nearby point defects. Simultaneously, a defect layer will be formed once the defect concentration is high enough, resulting in a blurry lattice fringe in the HRTEM images [31]. Hence, the intrinsic defective structure of α-MnO 2 was confirmed. Oxygen vacancies, as an important point defect in catalysts, play a prominent role in the catalytic reaction process, and the high oxygen vacancy concentration will result in a blurry lattice fringe, which can be reflected in the HRTEM images [32]. As shown in Figure 3, the EPR signal corresponding to g = 2.003 can be attributed to oxygen vacancies, and its signal intensity can represent the number of oxygen vacancies [27]. Therefore, more oxygen vacancies on α-MnO 2 than β-MnO 2 can be confirmed based on EPR, consistent with HRTEM analysis. results indicate the successful obtaining of the two kinds of MnO2 with specific crystal phases. The morphologies of MnO2 samples were characterized by a scanning electron microscopy (SEM), a transmission electron microscope (TEM), and a high-resolution TEM (HRTEM). As can be seen in Figure 2A, α-MnO2 showed a stacking-nanorod structure with an average length of about 330 nm. β-MnO2 ( Figure 2F) also showed a typical rod shape with a diameter near 50 nm and a length of ~1.2 μm. TEM showed consistent results with the SEM that α-MnO2 ( Figure 2B) and β-MnO2 ( Figure 2G) had similar nanorod-like structures as previously reported [30]. The well-identified periodic lattice fringes of 6.9 Å can be clearly observed in Figure 2C, corresponding to the interplanar distance of (110) facet of α-MnO2. Figure 2H exhibited the lattice fringes of 3.1 Å, which match the interplanar distance of the (110) facet of β-MnO2 well. Compared with the β-MnO2 samples, α-MnO2 showed more blurry lattice fringes, representing poor crystallinity, which also agrees well with the XRD patterns. In addition, the presence of defects was further demonstrated using the inverse Fast Fourier Transform (FFT) pattern ( Figure 2E,J). Significantly more lattice distortion can be clearly observed on the surface of α-MnO2 ( Figure 2D) (highlighted by red ovals), thus leading to more defects than β-MnO2 [10]. Besides, severe blurring of the lattice fringes was also detected on α-MnO2 than β-MnO2. Lattice distortion can be caused by nearby point defects. Simultaneously, a defect layer will be formed once the defect concentration is high enough, resulting in a blurry lattice fringe in the HRTEM images [31]. Hence, the intrinsic defective structure of α-MnO2 was confirmed. Oxygen vacancies, as an important point defect in catalysts, play a prominent role in the catalytic reaction process, and the high oxygen vacancy concentration will result in a blurry lattice fringe, which can be reflected in the HRTEM images [32]. As shown in Figure 3, the EPR signal corresponding to g = 2.003 can be attributed to oxygen vacancies, and its signal intensity can represent the number of oxygen vacancies [27]. Therefore, more oxygen vacancies on α-MnO2 than β-MnO2 can be confirmed based on EPR, consistent with HRTEM analysis.   The BET surface areas (SBET), and pore volumes of the two catalysts are shown in Figure 4A,B. It is reported that the different structures assembled by MnO6 octahedra in MnO2 will affect the related surface areas and pore volumes. β-MnO2 presented relatively low specific surface areas (12.76 m 2 g −1 ) and pore volumes (0.06 cm 3 g −1 ), whereas α-MnO2 showed higher specific surface areas (34.59 m 2 g −1 ) and pore volumes (0.13 cm 3 g −1 ). Moreover, the nitrogen adsorption-desorption isotherms of α-MnO2 and β-MnO2 displayed a type IV curve with H3-type hysteresis loops, indicating that both samples were mesoporous structures [33].

Catalytic Performance
In order to explore the activity of two catalysts on sulfur-containing volatile organic pollutants (S-VOCs), methyl mercaptan (CH3SH) was chosen as the model S-VOCs, and the catalytic activities of α-MnO2 and β-MnO2 are shown in Figure 5A. Two MnO2 samples exhibited significantly different catalytic performance in CH3SH catalytic reaction. α-MnO2 (74%) exhibited significantly better catalyst activity than β-MnO2 (3%) at 30 °C. The decreases of CH3SH conversion for α-MnO2 at 50 °C may be due to the desorption of CH3SH on the catalyst. As the temperature increased, the conversion of CH3SH reached 100% at 100 °C with both catalysts. Furthermore, the reaction rates of α-MnO2 and β-MnO2 at 50 °C were calculated based on the activity experiments. As shown in Figure 5B, α-MnO2 showed the CH3SH reaction rate of 2.9 × 10 −2 mol g −1 h −1 , this being ~4.1 times higher than the rates measured for and β-MnO2 at 50 °C, which was consistent with the results for the catalytic activity. The BET surface areas (S BET ), and pore volumes of the two catalysts are shown in Figure 4A,B. It is reported that the different structures assembled by MnO 6 octahedra in MnO 2 will affect the related surface areas and pore volumes. β-MnO 2 presented relatively low specific surface areas (12.76 m 2 g −1 ) and pore volumes (0.06 cm 3 g −1 ), whereas α-MnO 2 showed higher specific surface areas (34.59 m 2 g −1 ) and pore volumes (0.13 cm 3 g −1 ). Moreover, the nitrogen adsorption-desorption isotherms of α-MnO 2 and β-MnO 2 displayed a type IV curve with H 3 -type hysteresis loops, indicating that both samples were mesoporous structures [33].  The BET surface areas (SBET), and pore volumes of the two catalysts are shown in Figure 4A,B. It is reported that the different structures assembled by MnO6 octahedra in MnO2 will affect the related surface areas and pore volumes. β-MnO2 presented relatively low specific surface areas (12.76 m 2 g −1 ) and pore volumes (0.06 cm 3 g −1 ), whereas α-MnO2 showed higher specific surface areas (34.59 m 2 g −1 ) and pore volumes (0.13 cm 3 g −1 ). Moreover, the nitrogen adsorption-desorption isotherms of α-MnO2 and β-MnO2 displayed a type IV curve with H3-type hysteresis loops, indicating that both samples were mesoporous structures [33].

Catalytic Performance
In order to explore the activity of two catalysts on sulfur-containing volatile organic pollutants (S-VOCs), methyl mercaptan (CH3SH) was chosen as the model S-VOCs, and the catalytic activities of α-MnO2 and β-MnO2 are shown in Figure 5A. Two MnO2 samples exhibited significantly different catalytic performance in CH3SH catalytic reaction. α-MnO2 (74%) exhibited significantly better catalyst activity than β-MnO2 (3%) at 30 °C. The decreases of CH3SH conversion for α-MnO2 at 50 °C may be due to the desorption of CH3SH on the catalyst. As the temperature increased, the conversion of CH3SH reached 100% at 100 °C with both catalysts. Furthermore, the reaction rates of α-MnO2 and β-MnO2 at 50 °C were calculated based on the activity experiments. As shown in Figure 5B, α-MnO2 showed the CH3SH reaction rate of 2.9 × 10 −2 mol g −1 h −1 , this being ~4.1 times higher than the rates measured for and β-MnO2 at 50 °C, which was consistent with the results for the catalytic activity.

Catalytic Performance
In order to explore the activity of two catalysts on sulfur-containing volatile organic pollutants (S-VOCs), methyl mercaptan (CH 3 SH) was chosen as the model S-VOCs, and the catalytic activities of α-MnO 2 and β-MnO 2 are shown in Figure 5A. Two MnO 2 samples exhibited significantly different catalytic performance in CH 3 SH catalytic reaction. α-MnO 2 (74%) exhibited significantly better catalyst activity than β-MnO 2 (3%) at 30 • C. The decreases of CH 3 SH conversion for α-MnO 2 at 50 • C may be due to the desorption of CH 3 SH on the catalyst. As the temperature increased, the conversion of CH 3 SH reached 100% at 100 • C with both catalysts. Furthermore, the reaction rates of α-MnO 2 and β-MnO 2 at 50 • C were calculated based on the activity experiments. As shown in Figure 5B, α-MnO 2 showed the CH 3 SH reaction rate of 2.9 × 10 −2 mol g −1 h −1 , this being~4.1 times higher than the rates measured for and β-MnO 2 at 50 • C, which was consistent with the results for the catalytic activity. It is well known that the specific surface area plays a critical role in catalytic reactions. To eliminate its influence, the reaction rates with surface area normalization were calculated at different temperatures based on the data from the activity experiments. The results of the normalized reaction rates (rnorm, mol m −2 h −1 ) of CH3SH decomposition are shown in Figure 6A. The normalized reaction rates for α-MnO2 were obviously higher than those of β-MnO2 at 30, 50, 60 and 80 °C, which suggested that reactivity was not governed by the specific surface area. Turnover frequency (TOF) is essential for studying the intrinsic reactivity of catalysts. In this work, the TOF (h −1 ) was calculated based on oxygen vacancy concentration, and the TOF value for the CH3SH catalytic decomposition was conducted at 50 °C with 0.01 g of catalyst and was calculated within a low CH3SH conversion (1 h of reaction, below 15.0%). As displayed in Figure 6B, the α-MnO2 showed the highest TOF value of 0.14 h −1 , which was 1.8 times as that of the β-MnO2 (0.08 h −1 ), indicating that α-MnO2 has better catalytic performance for CH3SH.

Redox Capacity and Oxygen Species
To evaluate the reduction behaviors of MnO2 samples, H2-temperature-programmed reduction (TPR) was performed ( Figure 7A). For α-MnO2, the peaks at 289 and 309 °C corresponded to the reduction of Mn 4+  Mn 3+ and Mn 3+  Mn 2+ , respectively, and the 4+ 3+ 3+ It is well known that the specific surface area plays a critical role in catalytic reactions. To eliminate its influence, the reaction rates with surface area normalization were calculated at different temperatures based on the data from the activity experiments. The results of the normalized reaction rates (r norm , mol m −2 h −1 ) of CH 3 SH decomposition are shown in Figure 6A. The normalized reaction rates for α-MnO 2 were obviously higher than those of β-MnO 2 at 30, 50, 60 and 80 • C, which suggested that reactivity was not governed by the specific surface area. Turnover frequency (TOF) is essential for studying the intrinsic reactivity of catalysts. In this work, the TOF (h −1 ) was calculated based on oxygen vacancy concentration, and the TOF value for the CH 3 SH catalytic decomposition was conducted at 50 • C with 0.01 g of catalyst and was calculated within a low CH 3 SH conversion (1 h of reaction, below 15.0%). As displayed in Figure 6B, the α-MnO 2 showed the highest TOF value of 0.14 h −1 , which was 1.8 times as that of the β-MnO 2 (0.08 h −1 ), indicating that α-MnO 2 has better catalytic performance for CH 3 SH. It is well known that the specific surface area plays a critical role in catalytic reactions. To eliminate its influence, the reaction rates with surface area normalization were calculated at different temperatures based on the data from the activity experiments. The results of the normalized reaction rates (rnorm, mol m −2 h −1 ) of CH3SH decomposition are shown in Figure 6A. The normalized reaction rates for α-MnO2 were obviously higher than those of β-MnO2 at 30, 50, 60 and 80 °C, which suggested that reactivity was not governed by the specific surface area. Turnover frequency (TOF) is essential for studying the intrinsic reactivity of catalysts. In this work, the TOF (h −1 ) was calculated based on oxygen vacancy concentration, and the TOF value for the CH3SH catalytic decomposition was conducted at 50 °C with 0.01 g of catalyst and was calculated within a low CH3SH conversion (1 h of reaction, below 15.0%). As displayed in Figure 6B, the α-MnO2 showed the highest TOF value of 0.14 h −1 , which was 1.8 times as that of the β-MnO2 (0.08 h −1 ), indicating that α-MnO2 has better catalytic performance for CH3SH.

Redox Capacity and Oxygen Species
To evaluate the reduction behaviors of MnO2 samples, H2-temperature-programmed reduction (TPR) was performed ( Figure 7A). For α-MnO2, the peaks at 289 and 309 °C corresponded to the reduction of Mn 4+  Mn 3+ and Mn 3+  Mn 2+ , respectively, and the peaks around 291 and 317 °C for β-MnO2 were attributed to Mn 4+  Mn 3+ and Mn 3+  Mn 2+ , respectively [20,34]. The reduction temperature of α-MnO2 was lower than that of β-MnO2, indicating that the reduction of α-MnO2 is relatively faster. More importantly,

Redox Capacity and Oxygen Species
To evaluate the reduction behaviors of MnO 2 samples, H 2 -temperature-programmed reduction (TPR) was performed ( Figure 7A). For α-MnO 2 , the peaks at 289 and 309 • C corresponded to the reduction of Mn 4+ → Mn 3+ and Mn 3+ → Mn 2+ , respectively, and the peaks around 291 and 317 • C for β-MnO 2 were attributed to Mn 4+ → Mn 3+ and Mn 3+ → Mn 2+ , respectively [20,34]. The reduction temperature of α-MnO 2 was lower than that of β-MnO 2 , indicating that the reduction of α-MnO 2 is relatively faster. More importantly, the more remarkable reduction ability of α-MnO 2 means easier deoxygenation during hydrogen treatment, suggesting that oxygen migration is more likely to occur on its surface. Therefore, α-MnO 2 features stronger oxygen species mobility than β-MnO 2 .

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hydrogen treatment, suggesting that oxygen migration is more likely to occur on its surface. Therefore, α-MnO2 features stronger oxygen species mobility than β-MnO2. O2-TPD was conducted further to explore the oxygen species of the MnO2 catalysts. Figure 7B shows three desorption peaks related to oxygen species that can be observed on the MnO2. The low-temperature peak below 400 °C was ascribed to the chemisorbed active oxygen species on the surface (O − and O2 − ) [35]. The desorption peaks at 400-650 °C and 700-850 °C were related to the release of subsurface and bulk lattice oxygen species (O 2− ), respectively [36,37]. The desorption of surface oxygen at low temperature (<400 °C) plays the primary role as reactive oxygen species participating in the catalytic reaction [38]. Moreover, the lower temperature of the surface oxygen desorption peak means better low-temperature mobility of oxygen species. As depicted in Figure 7B, α-MnO2 showed a lower temperature at 146 °C of the surface oxygen desorption peak than β-MnO2 at 367 °C, indicating the better low-temperature mobility of oxygen species, which is in agreement with H2-TPR.
More bonding properties were discussed through Raman spectra ( Figure 8A). The peaks at 348 and 640 cm −1 corresponded to the Mn-O bending and the stretching vibration, respectively [39]. Significantly weaker and broader Raman peaks at around 640 cm −1 were detected for α-MnO2 than β-MnO2, suggesting lower crystallinity and more defects due to the lattice distortion [40]. To evaluate the strength of the Mn-O bond, the bond force constant (k) was calculated from Hooke's law [41,42]   O 2 -TPD was conducted further to explore the oxygen species of the MnO 2 catalysts. Figure 7B shows three desorption peaks related to oxygen species that can be observed on the MnO 2 . The low-temperature peak below 400 • C was ascribed to the chemisorbed active oxygen species on the surface (O − and O 2 − ) [35]. The desorption peaks at 400-650 • C and 700-850 • C were related to the release of subsurface and bulk lattice oxygen species (O 2− ), respectively [36,37]. The desorption of surface oxygen at low temperature (<400 • C) plays the primary role as reactive oxygen species participating in the catalytic reaction [38]. Moreover, the lower temperature of the surface oxygen desorption peak means better low-temperature mobility of oxygen species. As depicted in Figure 7B, α-MnO 2 showed a lower temperature at 146 • C of the surface oxygen desorption peak than β-MnO 2 at 367 • C, indicating the better low-temperature mobility of oxygen species, which is in agreement with H 2 -TPR.
More bonding properties were discussed through Raman spectra ( Figure 8A). The peaks at 348 and 640 cm −1 corresponded to the Mn-O bending and the stretching vibration, respectively [39]. Significantly weaker and broader Raman peaks at around 640 cm −1 were detected for α-MnO 2 than β-MnO 2 , suggesting lower crystallinity and more defects due to the lattice distortion [40]. To evaluate the strength of the Mn-O bond, the bond force constant (k) was calculated from Hooke's law [41,42] using the following equation:

Identification of the Role of Oxygen Vacancies in CH 3 SH Degradation
The surface elemental composition and chemical state of these MnO 2 samples were identified by XPS. The XPS spectra of Mn 2p 3/2 of the samples are shown in Figure 9. The peaks corresponding to binding energies at 642.7, 641.7 and 640.6 eV can be attributed to Mn 4+ , Mn 3+ and Mn 2+ , respectively [43,44]. It is noteworthy that the binding energy corresponding to different valences of Mn were slightly different in both MnO 2 samples, indicating that crystal phase structure has a certain effect on the electron density of the MnO 2 surface, which is related to the degree of charge imbalance, oxygen vacancies, as well as the relative content of Mn 2+ , Mn 3+ and Mn 4+ [45]. Specifically, the oxygen vacancy will be generated to maintain electrostatic balance with the increasing Mn 2+ and Mn 3+ proportion, and the proportion of low-valence Mn is generally regarded as an indicator of surface oxygen vacancies [44]. As shown in Table 1 and Figure 9A, the proportion of the low valence Mn (Mn 2+ + Mn 3+ ) showed α-MnO 2 (41.62%) > β-MnO 2 (37.74%). Besides, the average oxidation state (AOS) of MnO 2 was calculated according to the formula of AOS = 8.956 − 1.126 ∆E [46], which was based on the size of Mn 3s multiple splitting (∆E) in Mn 3s XPS spectra ( Figure 10A). The AOS values of the Mn element in α-MnO 2 and β-MnO 2 were calculated to be 3.57 and 3.75, respectively. In previous studies, lower AOS of MnO 2 was also able to indicate more surface oxygen vacancies [47]. Therefore, it can be inferred that α-MnO 2 has a greater surface oxygen vacancy density than β-MnO 2 (in agreement with HETEM and EPR). and 700-850 °C were related to the release of subsurface and bulk lattice oxygen species (O 2− ), respectively [36,37]. The desorption of surface oxygen at low temperature (<400 °C) plays the primary role as reactive oxygen species participating in the catalytic reaction [38]. Moreover, the lower temperature of the surface oxygen desorption peak means better low-temperature mobility of oxygen species. As depicted in Figure 7B, α-MnO2 showed a lower temperature at 146 °C of the surface oxygen desorption peak than β-MnO2 at 367 °C, indicating the better low-temperature mobility of oxygen species, which is in agreement with H2-TPR.
More bonding properties were discussed through Raman spectra ( Figure 8A). The peaks at 348 and 640 cm −1 corresponded to the Mn-O bending and the stretching vibration, respectively [39]. Significantly weaker and broader Raman peaks at around 640 cm −1 were detected for α-MnO2 than β-MnO2, suggesting lower crystallinity and more defects due to the lattice distortion [40]. To evaluate the strength of the Mn-O bond, the bond force constant (k) was calculated from Hooke's law [41,42]

Identification of the Role of Oxygen Vacancies in CH3SH Degradation
The surface elemental composition and chemical state of these MnO2 samples were identified by XPS. The XPS spectra of Mn 2p3/2 of the samples are shown in Figure 9. The peaks corresponding to binding energies at 642.7, 641.7 and 640.6 eV can be attributed to Mn 4+ , Mn 3+ and Mn 2+ , respectively [43,44]. It is noteworthy that the binding energy corresponding to different valences of Mn were slightly different in both MnO2 samples, indicating that crystal phase structure has a certain effect on the electron density of the MnO2 surface, which is related to the degree of charge imbalance, oxygen vacancies, as well as the relative content of Mn 2+ , Mn 3+ and Mn 4+ [45]. Specifically, the oxygen vacancy will be generated to maintain electrostatic balance with the increasing Mn 2+ and Mn 3+ proportion, and the proportion of low-valence Mn is generally regarded as an indicator of surface oxygen vacancies [44]. As shown in Table 1 and Figure 9A, the proportion of the low valence Mn (Mn 2+ + Mn 3+ ) showed α-MnO2 (41.62%) > β-MnO2 (37.74%). Besides, the average oxidation state (AOS) of MnO2 was calculated according to the formula of AOS = 8.956 − 1.126 ΔE [46], which was based on the size of Mn 3s multiple splitting (ΔE) in Mn 3s XPS spectra ( Figure 10A). The AOS values of the Mn element in α-MnO2 and β-MnO2 were calculated to be 3.57 and 3.75, respectively. In previous studies, lower AOS of MnO2 was also able to indicate more surface oxygen vacancies [47]. Therefore, it can be inferred that α-MnO2 has a greater surface oxygen vacancy density than β-MnO2 (in agreement with HETEM and EPR).      Figure 9A showed that α-MnO2 had more low valence Mn (Mn 2+ + Mn 3+ ) than β-MnO2, which also implies the easier release of surface oxygen species, consistent with the results of H2-TPR and O2-TPD. Comparison of the Mn 2p3/2 spectra before and after the reaction ( Figure 9B) of MnO2 with CH3SH showed that the valence state of Mn in both samples changed obviously, suggesting the electron transfer during the reaction. For both samples, Mn 4+ decreased, and Mn 2+ and Mn 3+ increased, proving that the high-valent Mn (IV) was reduced by gaining electrons during the reaction. After the reaction, the Mn 2+ +Mn 3+ /Mn 4+ of α-MnO2 increased by 1.51 and that of β-MnO2 by 1.39, and AOS decreased by 0.74 for α-MnO2 and that of β-MnO2 decreased by 0.5 ( Figure 10B), testifying that α-MnO2 was reduced to a greater extent by gaining more electrons than β-MnO2, which can well match H2-TPR results. It is noteworthy that higher AOS usually indicates a stronger electron-gaining ability of the catalyst because of the presence of more highvalent atoms, however, α-MnO2 exhibited a stronger electron-gaining ability in the reaction with CH3SH, suggesting that oxygen vacancies play a more important role in catalyzing CH3SH comparing to the high-valent Mn. This may explain the fact that chemisorption is the rate-limiting step for electron transfer, and more surface oxygen vacancies provide more surface adsorption sites for CH3SH.
In addition, the reaction between CH3SH and the catalyst could change the electronic  Figure 9A showed that α-MnO 2 had more low valence Mn (Mn 2+ + Mn 3+ ) than β-MnO 2 , which also implies the easier release of surface oxygen species, consistent with the results of H 2 -TPR and O 2 -TPD. Comparison of the Mn 2p 3/2 spectra before and after the reaction ( Figure 9B) of MnO 2 with CH 3 SH showed that the valence state of Mn in both samples changed obviously, suggesting the electron transfer during the reaction. For both samples, Mn 4+ decreased, and Mn 2+ and Mn 3+ increased, proving that the high-valent Mn (IV) was reduced by gaining electrons during the reaction. After the reaction, the Mn 2+ +Mn 3+ /Mn 4+ of α-MnO 2 increased by 1.51 and that of β-MnO 2 by 1.39, and AOS decreased by 0.74 for α-MnO 2 and that of β-MnO 2 decreased by 0.5 ( Figure 10B), testifying that α-MnO 2 was reduced to a greater extent by gaining more electrons than β-MnO 2 , which can well match H 2 -TPR results. It is noteworthy that higher AOS usually indicates a stronger electron-gaining ability of the catalyst because of the presence of more high-valent atoms, however, α-MnO 2 exhibited a stronger electron-gaining ability in the reaction with CH 3 SH, suggesting that oxygen vacancies play a more important role in catalyzing CH 3 SH comparing to the high-valent Mn. This may explain the fact that chemisorption is the rate-limiting step for electron transfer, and more surface oxygen vacancies provide more surface adsorption sites for CH 3 SH.
In addition, the reaction between CH 3 SH and the catalyst could change the electronic environment of the catalyst. During the reaction, as the ratio of Mn 2+ and Mn 3+ increased, weaker Mn-O bonds were continuously formed and broken, leading to deoxygenation and further generation of oxygen vacancies to maintain electrostatic equilibrium, which may provide new sites for the reaction, and these desorbed oxygen species may favor the catalytic oxidation of CH 3 SH as well.
The XPS spectra of O 1s of the samples are shown in Figure 11. Peaks with binding energies at 529-529.8, 530.9-532 and 533 eV in the XPS spectra of O 1s of the MnO 2 samples (Figure 11) can be attributed to lattice oxygen (O latt ) and surface adsorption oxygen (O ads ), and surface hydroxyl oxygen (O adsO-H ), respectively [49][50][51]. The molar ratio of O ads /O latt is shown in Table 2 and follows the order of α-MnO 2 (0.65) > β-MnO 2 (0.35). O ads was generally considered the most reactive oxygen species in the catalytic reaction and capable of participating in the catalytic oxidation of VOCs in previous reports [52,53]. The oxygen species changes before and after the reaction are shown in Figure 11B and Table 2. The O latt and O ads for both two materials decreased and increased, respectively, suggesting the migration of O latt to form O ads during the reduction of Mn. Obviously, α-MnO 2 formed more O ads after the reaction, which corresponds to its greater degree of reduction, also implying that α-MnO 2 has a stronger catalytic capacity.

023, 13,
11 of 16 ( Figure 11) can be attributed to lattice oxygen (Olatt) and surface adsorption oxygen (Oads), and surface hydroxyl oxygen (OadsO-H), respectively [49][50][51]. The molar ratio of Oads/Olatt is shown in Table 2 and follows the order of α-MnO2 (0.65) > β-MnO2(0.35). Oads was generally considered the most reactive oxygen species in the catalytic reaction and capable of participating in the catalytic oxidation of VOCs in previous reports [52,53]. The oxygen species changes before and after the reaction are shown in Figure 11B and Table 2. The Olatt and Oads for both two materials decreased and increased, respectively, suggesting the migration of Olatt to form Oads during the reduction of Mn. Obviously, α-MnO2 formed more Oads after the reaction, which corresponds to its greater degree of reduction, also implying that α-MnO2 has a stronger catalytic capacity.

Product Detection during the Reaction
The main gas phase products were monitored quantitatively to better understand the

Product Detection during the Reaction
The main gas phase products were monitored quantitatively to better understand the reaction process of CH 3 SH over two different catalysts. As displayed in Figure 12, the decomposition of CH 3 SH at different temperatures corresponded to the production of CH 3 SSCH 3 . Meanwhile, the concentration of CH 3 SH during the reaction showed an excellent correlation with the concentration of CH 3 SSCH 3 , indicating that CH 3 SSCH 3 was the main gas-phase product. At 150 • C, CH 3 SH was completely decomposed for both MnO 2 catalysts, consistent with the thermodynamic theory that catalytic reactions proceed easier at higher temperatures. The yield of CH 3 SSCH 3 gradually decreased when T > 100 • C, which may be due to the further catalytic oxidation of CH 3 SSCH 3 at higher temperatures.  Figure 13 shows the changes of S 2p before and after the reaction, which was used to detect the solid-phase intermediates in the reaction process. No S species were detected on two catalysts before the reaction. In contrast, significant amounts of S species were detected on both samples after the reaction, indicating that some sulfur-containing products were adsorbed on the catalyst surface. As shown in Figure 13B, three peaks at 163.2, 167.9, and 169.2 eV corresponding to S 0 , SO3 2− (S 4+ ), and SO4 2− (S 6+ ), respectively [30,54], which all showed higher valence than S 2-from CH3SH, indicating the oxidation of S during the reaction. It is worth noting that the catalytic experiments in CH3SH were performed under a nitrogen atmosphere, so it can be concluded that the O in the S-O species was mainly derived from the Oads of MnO2. This illustrated the Oads involvement in the catalytic reaction of CH3SH. SO3 2− and SO4 2− were mainly retained on the manganese dioxide surface in the form of MnSO3 and MnSO4, implying that chemisorption was a prerequisite for the decomposition of CH3SH on MnO2. Notably, reacted α-MnO2 showed a higher proportion of SO4 2− (S 6+ ) (26.31%) than β-MnO2 (12.56%) ( Table 3 and Figure 13B), suggesting a greater degree of S oxidation, which corresponds to a greater reduction of Mn 4+ after the reaction (Table 1 and Figure 9B). Furthermore, SO4 2− requires more oxygen to be coordinated with S than SO3 2− , so a higher proportion of SO4 2− production requires more Oads to participate in the reaction. Correspondingly, the H2-TPR, O2-TPD, and XPS analysis demonstrated more Oads and better surface oxygen mobility for α-MnO2 than β-MnO2.  Figure 13 shows the changes of S 2p before and after the reaction, which was used to detect the solid-phase intermediates in the reaction process. No S species were detected on two catalysts before the reaction. In contrast, significant amounts of S species were detected on both samples after the reaction, indicating that some sulfur-containing products were adsorbed on the catalyst surface. As shown in Figure 13B, three peaks at 163.2, 167.9, and 169.2 eV corresponding to S 0 , SO 3 2− (S 4+ ), and SO 4 2− (S 6+ ), respectively [30,54], which all showed higher valence than S 2from CH 3 SH, indicating the oxidation of S during the reaction. It is worth noting that the catalytic experiments in CH 3 SH were performed under a nitrogen atmosphere, so it can be concluded that the O in the S-O species was mainly derived from the O ads of MnO 2 . This illustrated the O ads involvement in the catalytic reaction of CH 3 SH. SO 3 2− and SO 4 2− were mainly retained on the manganese dioxide surface in the form of MnSO 3 and MnSO 4 , implying that chemisorption was a prerequisite for the decomposition of CH 3 SH on MnO 2 . Notably, reacted α-MnO 2 showed a higher proportion of SO 4 2− (S 6+ ) (26.31%) than β-MnO 2 (12.56%) ( Table 3 and Figure 13B), suggesting a greater degree of S oxidation, which corresponds to a greater reduction of Mn 4+ after the reaction (Table 1 and Figure 9B). Furthermore, SO 4 2− requires more oxygen to be coordinated with S than SO 3 2− , so a higher proportion of SO 4 2− production requires more O ads to participate in the reaction. Correspondingly, the H 2 -TPR, O 2 -TPD, and XPS analysis demonstrated more O ads and better surface oxygen mobility for α-MnO 2 than β-MnO 2 .
Based on the above experimental and characterization analysis, the catalytic mechanism of CH 3 SH by α-MnO 2 and β-MnO 2 can be inferred in Figure 14. CH 3 SH was first chemisorbed on the MnO 2 surface and subsequently underwent a single electron transfer to form CH 3 S·, and then the two CH 3 S· were coupled to form CH 3 SSCH 3 . Based on the formation of S-S bonds, it is speculated that the single electron transfer occurs on S, suggesting that the chemisorption may be through the formation of Mn-S bonds. Moreover, more lattice oxygen was released during the reduction of Mn, which was involved in the further catalytic oxidation of S-containing species to produce SO 3 2− and SO 4 2− , and may further form new oxygen vacancies to support more active sites. Although β-MnO 2 enjoys a higher AOS based on the proportion of high valence Mn, α-MnO 2 showed better catalytic activity due to more oxygen vacancies and stronger oxygen mobility.
ing the reaction. It is worth noting that the catalytic experiments in CH3SH were performed under a nitrogen atmosphere, so it can be concluded that the O in the S-O species was mainly derived from the Oads of MnO2. This illustrated the Oads involvement in the catalytic reaction of CH3SH. SO3 2− and SO4 2− were mainly retained on the manganese dioxide surface in the form of MnSO3 and MnSO4, implying that chemisorption was a prerequisite for the decomposition of CH3SH on MnO2. Notably, reacted α-MnO2 showed a higher proportion of SO4 2− (S 6+ ) (26.31%) than β-MnO2 (12.56%) ( Table 3 and Figure 13B), suggesting a greater degree of S oxidation, which corresponds to a greater reduction of Mn 4+ after the reaction (Table 1 and Figure 9B). Furthermore, SO4 2− requires more oxygen to be coordinated with S than SO3 2− , so a higher proportion of SO4 2− production requires more Oads to participate in the reaction. Correspondingly, the H2-TPR, O2-TPD, and XPS analysis demonstrated more Oads and better surface oxygen mobility for α-MnO2 than β-MnO2.   Based on the above experimental and characterization analysis, the catalytic mechanism of CH3SH by α-MnO2 and β-MnO2 can be inferred in Figure 14. CH3SH was first chemisorbed on the MnO2 surface and subsequently underwent a single electron transfer to form CH3S·, and then the two CH3S· were coupled to form CH3SSCH3. Based on the formation of S-S bonds, it is speculated that the single electron transfer occurs on S, suggesting that the chemisorption may be through the formation of Mn-S bonds. Moreover, more lattice oxygen was released during the reduction of Mn, which was involved in the further catalytic oxidation of S-containing species to produce SO3 2− and SO4 2− , and may further form new oxygen vacancies to support more active sites. Although β-MnO2 enjoys a higher AOS based on the proportion of high valence Mn, α-MnO2 showed better catalytic activity due to more oxygen vacancies and stronger oxygen mobility. Figure 14. Reaction mechanism of CH3SH decomposition over MnO2 catalysts. Figure 14. Reaction mechanism of CH 3 SH decomposition over MnO 2 catalysts.

Conclusions
In this paper, MnO 2 catalysts with similar morphology but different crystal structures (α-MnO 2 and β-MnO 2 ) were successfully prepared, and the effects of the physicochemical properties on the catalytic activities were systematically investigated. Both Mn-based catalysts showed significant removal of CH 3 SH at 150 • C, achieving complete conversion, whereas α-MnO 2 exhibited significantly better catalytic activity compared to β-MnO 2 at a lower temperature (T < 100 • C) under a GHSV of 9000 mL g −1 h −1 . Coupled with O 2 -TPD, H 2 -TPR, Raman spectra, XPS, EPR, and HRTEM, it was demonstrated that α-MnO 2 has more oxygen vacancies, stronger surface oxygen migration ability, and better redox properties, which can be favorable for CH 3 SH decomposition. The readily released lattice oxygen during the reaction promoted further oxidative decomposition of S-containing species. The intermediate products of the solid and gas phases were determined as CH 3 SSCH 3 and the S 0 , SO 3 2− , and SO 4 2− , respectively. The catalytic mechanism was further proposed as the oxygen vacancies on MnO 2 provided active sites for the adsorption of CH 3 SH, facilitating the electron transfer of MnO 2 with CH 3 SH, and the oxygen species derived from the Mn surface were further involved in the CH 3 SH catalytic oxidation. The findings of this study are essential for broadening the application of Mn-based catalysts in the removal of S-VOCs and providing new insights into the mechanism of interfacial reactions between VOCs and metal-based catalysts.